The subject matter of this invention is oral impression trays, and devices and materials for making oral impression trays in situ.
In the prior art, oral impressions are taken by the dentist as a starting point in the fabrication of a variety of restorations and dental appliances. A number of impression material systems are available to meet the needs and varied skills of the dentists. Included in the list of available materials are a variety of elastomeric compositions based upon polysulfides, addition silicones, condensation silicones, polyethers and other elastomers. These materials typically are two-part systems, consisting of a "base" composition and a "catalyst" composition, which, when mixed, convert the fluid resin to an elastomer. When properly proportioned and mixed, the materials are carried into the oral cavity via some type of "tray." The trays may be reusable, of metal or plastic construction, and are designed to approximate the contour of either the mandibular or maxillary arch. They are usually provided in a series of sizes to permit a close approximation of fit to the oral area under consideration.
A similar variety of cheaper, less rigid disposable trays are also available to the dentist. These, again, only approximately fit the dental arch and, in general, are less structurally strong and rigid. All of these "standard" trays give only an approximate "fit" over the oral area of which an impression is needed. In some areas the trays may be nearly touching the oral tissues, and in other areas a considerable space may exist between the tray and the oral tissues. The variations in spacing result in a correspondingly varying bulk of impression material within the trays. This variation in material bulk contributes to inaccuracies in the oral impression and to waste of expensive impression material. Variations in tissue compression and variations in polymerization shrinkage are among the factors causing the inaccurate impressions.
In an effort to better control the tissue reactions and to optimize the stresses induced by the polymerization shrinkage of the impression material, the more discriminating dentist will use custom-made trays for taking impressions. This insures improved impression accuracy and minimizes future problems and remakes. Currently, custom trays are produced by an indirect (extra-oral) technique, as follows:
1. A "snap" impression is taken of the oral structure using a cheap impression material. The greatest accuracy is neither required nor sought.
2. An artificial stone model is poured into the "snap" impression and allowed to harden for approximately one hour. This model is thus a positive of the oral structure and is used for bench (as distinguished from in situ) construction of an oral impression tray.
3. The tray is made of a room-temperature curing plastic, usually methyl methacrylate. This involves cutting a suitable "spacer" and placing it over the positive model in the area of which an impression is required. This "spacer" insures a reasonably uniform space for the final impression material and is removed before using the tray. A handle may also be formed on the tray to facilitate handling and removal of the final impression from the oral cavity. The complete tray, which is thus a spaced negative of the positive model, is removed from the model upon completion of polymerization. It is trimmed, cleaned, separated from the spacer, and coated with an adhesive to help retain the final impression material. The tray is now ready for taking the impression.